Geologia e petrografia das rochas ígneas do Cerro dos Martins, Caçapava do Sul, RS

Abstract

The Cerro dos Martins district, located approximately 20 Km south of the city of Caçapava do Sul, has a homonymous high topographic, where in its surroundings is an important cupriferous deposit of the state of Rio Grande do Sul. Rocks of the Camaquã Basin occur in the region, belonging to the Bom Jardim Group, and an intrusive dioritic body (Cerro dos Martins Diorite), elongated in the NW - SE direction, associated with dikes of the same orientation. These rocks intrude volcanic and subvolcanic rocks of the Hilario Formation. Obtained ages from previous studies link the Cerro dos Martins Diorite to the Acampamento Velho volcanism (550 Ma, through Pb - Pb via evaporation). However, its location and form of occurrence along the enclosing rocks raise doubts regarding this link. Through petrography and structural relations sought to characterize the igneous rocks found near the Cerro dos Martins. The rocks described were separated into three groups, characterized as the first, second and third magmatic events, which went through two distinct hydrothermal episodes. Through petrographic comparisons, rocks belonging to the first (volcanic rocks) and second (subvolcanic rocks) magmatic events were associated with Hilario volcanism, which were affected by a hydrothermal episode, denominated first hydrothermal episode, rich in carbonates resulting from activities concomitant with volcanism or subsequent volcanic episodes of the Camaquã Basin. The third magmatic event is marked by the Cerro dos Martins Diorite, which occurs associated with dikes, sometimes cumulates, of NW - SE orientation. The rocks belonging to the third magmatic event were linked to the Serra Geral Formation (Cretaceous), in which the obtained ages performed in previous studies may be the result of zircons inherited from the neoproterozoic rocks. The intrusion of these rocks generated a strong hydrothermalism, denominated second hydrothermal episode, which affected the rocks of the first two magmatic events. Also, the rocks of the second magmatic event may have been directly affected by the intrusion of the Cerro dos Martins Diorite, causing contact metamorphism that permeated the albite-epidote-hornfels facies.